Sinn Fein has been accused of "doing austerity" for the Conservatives and of handing back hard-won welfare reform powers to Westminster.
The SDLP attack came as the Assembly took the first step yesterday in paving the way for London to implement the massive benefits changes - cushioned by the Executive's £585m top-up fund over the next four years.
Three of the five Executive parties failed to delay a special debate giving the green light for the Government to pass the necessary legislation.
In a six-hour session the day after the launch of the SF/DUP 'Fresh Start' document, new SDLP deputy leader Fearghal McKinney said: "We are being asked to diminish aspects of devolution that the SDLP fought hard to achieve. We reject that.
"We are being asked to hand over powers to the Tories - or Thatchers' children, as Martin McGuinness has called them. Sinn Fein are doing Tory austerity by allowing the British Government to implement reform."
Former SDLP minister Alex Attwood added: "How dare anybody reduce this chamber to a postbox after the years of democratic struggle and the pain and grief suffered by our people. How dare they take away from the rest of us the democratic mandate."
But Sinn Fein's Conor Murphy replied: "Everything changed with the election in May of a single-party Conservative government, which was even more hell-bent on pursuing austerity.
"We felt we had a responsibility to deal with the welfare issue, to use the resources available to create a safety net for deserving and vulnerable people in society. We all accepted that the tax credit issue changed that."
His colleague Alex Maskey challenged the other parties to produce alternatives, arguing it was important to "scotch the idea" that every welfare recipient can be protected from cuts.
What changed, he said, was that the people of Britain elected a Tory government.
Earlier, the three smaller parties fell behind a proposal from the TUV's Jim Allister, who said MLAs had not been given sight of the legislative Bill they were being urged to approve.
"It might be available to some clique, but it is not available to members of this house," Mr Allister said. "Why the rush? This house does itself no favours in its standing if it nods through something that it hasn't seen." Ulster Unionists, the SDLP and Alliance supported Mr Allister's suggestion to shift the debate to next Tuesday, but it was defeated by 58 votes to 33.
Social Development Minister Mervyn Storey insisted the bill had been published and that it was necessary to push ahead with the legislative consent motion so the issue could be dealt with before the current Assembly term ends in March.
Sinn Fein's Caitriona Ruane called Mr Allister's proposal a "gimmick" and argued it was time to get on with the debate on this "historic agreement".